Add documentation about configuring audit options

We don't have any docs on how to configure the options of an audit in a config.

ex:
```js
{
extends: 'default',
audits: [
{path: 'screenshot-thumbnails', options: {numberOfThumbnails: 10}},
],
}
```

Start by locating Lighthouse's existing documentation for configuration and audits; the issue provides the relevant config shape and an options example. Document how audit options are configured, including the shown screenshot-thumbnails example. Done means users can find clear guidance for configuring audit options in a config.

Written by the indexing model from the issue text.